This appears to be a rather unique bug that only occurs in select situations, but it's definitely repeatable.
In a comp, make a new plane (any color) that's 1920x100. In its Transform properties, unlock the scale link, and scale the plane only on its X axis. At around 19% scale, the top of the plane will begin to become transparent. At around 2%, this transparency will spread to about the top two-thirds of the plane. It won't be fully transparent at the topmost edge, but it definitely won't be fully opaque as it should be.
While messing with other layers, or sometimes just switching to another program and back, this semi-transparent behavior appears to go away. However, I tested various single-frame exports, and the export maintains the transparency despite how it may look in the view.
Scaling a full 1920x1080 plane in the same way doesn't trigger this behavior, though for what I'm doing (creating a lower-third element) I need the plane's height to be fixed from the beginning to match other elements in my comp. I've managed to work around this temporarily by using a 100x100 plane, but this edge transparency behavior still shouldn't be happening when any plane is scaled.
This is happening in the latest release of HitFilm Express 2017 on my Mac. System specs:
Mac Mini (Late 2014)
Mac OS X 10.10.5
2.6 GHz Intel Core i5
8 GB 1600 MHz DDR3
Intel Iris 1536 MB